This taxon was previously considered a subspecies of N. concolor and N. leucogenys (M. Richardson pers. comm.). The limits of the distribution of this species are unclear, especially with regards to N. siki. The identity of the gibbons in a large area covering parts of central Viet Nam, southern Lao and northeastern Cambodia is unclear, as these gibbons differ in their song from both N. gabriellae in the south and N. siki in the north, but phenotypically resemble N. gabriellae (Geissmann et al. 2000, Konrad and Geissmann 2006, T. Geissmann pers. comm., 2007). According to Delacour (1951) and Groves (1972), this species may possibly interbreed with N. siki in Saravane and Savannakhet, Lao (Geissmann et al. 2000).
Here, gibbons that, at least phenotypically (coloration), look like N. gabriellae, are included in this species. Phenotype information for the type locality of N. siki (Thua Luu, Thua Thien Hue province, central Viet Nam) is contradictory (Geissmann et al. 2000), but these gibbons are tentatively identified as N. gabriellae as well.

Justification:
This species is considered Endangered based on an estimated population reduction of over 50% when considering the past 45 years (three generations) due primarily to hunting and habitat loss. A close monitoring of this species is required given predicated likely rates of both habitat loss and hunting in the future.

Traditionally, the range of N. gabriellae includes northeastern Cambodia, south of Ratanakari province, and southern Viet Nam, south of Bach Ma. The range here extends further to the north to include animals that, at least phenotypically (coloration), are N. gabriellae, to include southern Lao PDR, as far north as Savannakhet and to Thua Thien Hue province (and possibly Quang Tri province) in central Viet Nam (Geissmann et al. 2000).

It is likely that this species is the most common of the crested gibbons in Viet Nam, although this is difficult to assess given the uncertainty of the identity of animals in the northern part of the range (Geissmann et al. 2000). Brickle et al. (1998) reported that it was fairly common in some areas of Dak Lak province, and the Lam Dong Plateau seems to support a relatively large population of this species (Geissmann et al. 2000). In Bach Ma National Park (central Viet Nam), eight groups were recorded during a survey covering one 6 km2 of the park (Geissmann et al. 2003). In Cat Tien National Park the population has been estimated at 150 groups and around 500 individuals (Hao et al. 2005). Based on a status report, Geissmann et al. (2003) recorded 15 localities where traditional N. gabriellae should occur, of which five no longer held any populations. There were an additional nine localities for the questionable N. gabriellae area, and gibbons no longer occurred in three of these. These must have been relatively recent losses. Recent disappearance of individual groups (for undetermined reasons) was also reported for Bach Ma National Park (Tallents et al. 2001), and disappearance of individual groups as a result of habitat clearance was reported for Nam Cat Tien (Geissmann 1995).

In Cambodia, in Seima Biodiversity Conservation Area, the population is estimated at between 1,300 and 1,700 groups (Rawson and Clements in prep.), but including the surrounding forested areas the total population may be twice this size (Traeholt et al. 2005). Other estimates include: about 850 groups in Snoul Wildlife Sanctuary; about 360 groups in Phnom Prich Wildlife Sanctuary estimated; 330 groups in Nam Lyr Wildlife Sanctuary; 5,750 groups in Virachey National Park, and 1,100 groups in the Pheapimex concession (Traeholt et al. 2005).

In Lao PDR, high gibbon densities have been reported from Xe Pian National Biodiversity Conservation Area (Duckworth et al. 1995, 1999). There is an estimated 400-6,720 groups from Xe Pian National Biodiversity Conservation Area (Duckworth et al. 1995). The large Xe Pian-Dong Hua Sao National Biodiversity Conservation Areas population is of major global significance for gibbon conservation (Duckworth et al. 1999).

During a survey of about 6 km2 of Bach Ma National Park (Thua Thien Hue province, central Viet Nam) where forests occupy 220 km², a density of about 1.3 goups/km2 was estimated (Geissmann et al. 2007). Among six provinces from Thua Thien Hue (central Viet Nam) to Thanh Hoa (southern part of north Viet Nam), Dak Rong Nature Reserve (Quang Tri province, central Viet Nam) was identified as one locality with the highest gibbon density, with an estimated density of only 0.06 groups/km2 (Nguyen Manh Ha et al. 2005). Estimated population densities in Cambodia range from 0.00 to 3.73 groups/km2 (n = 15), with an average of 1.47 groups/km2 (Traeholt et al. 2005).

This species is found in tall evergreen and semi-evergreen forest (Geissman et al. 2000), although it probably ranges into other forest types (like mixed bamboo and woodland forest) adjacent to these, and may also occur in riverine and gallery forest associations. In Bach Ma National Park (central Viet Nam), gibbons appeared to live in lowland evergreen forest at altitudes between 400 and 800 m, although the park area also includes forest areas at higher altitudes (Geissmann et al. 2007). In Lao PDR, gibbons are found from the Mekong plains up to at least 1,550 in the Phou Ahyon area, and 1,650 m in Phou Louey National Biodiversity Conservation Area (Duckworth et al. 1999), but the species is scarce above an altitude of 1,500 m (Eames and Robson, 1993).

Like other gibbons, yellow-cheeked crested gibbons are arboreal and diurnal. Average group size is on the order 3-5 individuals. They feed mainly on fruit and leaves (Traeholt et al. 2005). Home range sizes in Nam Cat Tien National Park range from less than 30 ha in evergreen forest to up to 100 ha in bamboo forest (Traeholt et al. 2005).

The major threat in Cambodia and Viet Nam is hunting for the pet trade, although in Lao PDR hunting takes place mainly for food. Areas in southern Viet Nam have been heavily degraded by the spraying of aerial defoliant, agricultural encroachment, and logging, though this species appears to survive in moderately disturbed forest, as suggested by its continued presence in Cat Tien National Park and Dak Uyn Sate Forest Enterprise (Geissmann et al. 2000).

This species is listed in CITES Appendix I. In Viet Nam it is listed on Appendix 1B of Decree 32, 2006. In Cambodia, yellow-cheeked gibbons have been recorded from several protected areas, including Snoul Wildlife Sanctuary, Seima Biodiversity Conservation Area, Phnom Prich Wildlife Sanctuary, Nam Lyr Wildlife Sanctuary, and Virachey National Park (Traeholt et al. 2005). In Lao PDR, they are present in effectively all protected areas within their range (Duckworth et al. 1999). In Viet Nam, Bach Ma National Park, Cat Tien National Park, Bu Gia Map National Park and Nui Chua National Park hold important populations (T. Geissmann et al. pers comm.).

Recommended conservation actions for this species include: regulation of hunting and wildlife trade; minimization of habitat disturbance; and research and field surveys throughout the range, specifically sound recordings, genetic analysis and photographic recordings to help better define the distribution area (Geissmann et al. 2000). The species is the second-most common species of crested gibbons (genus Nomascus) maintained in zoos (Gibbon Network 2006; Moisson and Baudier 2005; Varsik 2000).

In-Place Research, Monitoring and PlanningIn-Place Land/Water Protection and Management Occur in at least one PA:YesIn-Place Species Management Subject to ex-situ conservation:YesIn-Place Education Included in international legislation:Yes Subject to any international management/trade controls:Yes